Recently, constant ON-time converters are widely used per their fast transient response, simple structure and smooth operation mode switching.
FIG. 1 schematically illustrates a conventional constant ON-time converter 100. Constant ON-time converter 100 comprises an ON-time control circuit 101, a comparison circuit 102, a logic circuit 103, a switching circuit 104 and a feedback circuit 106. Switching circuit 104 comprises a switch and switching circuit 104 is configured to provide output signal OUT from input signal IN via turning ON and OFF the switch. ON-time control circuit 101 is configured to provide an ON-time period control signal COT to control an ON-time period of the switch. Feedback circuit 106 comprises an output configured to provide a feedback signal VFB indicating output signal OUT. Comparison circuit 102 comprises an output configured to provide a comparing signal via comparing feedback signal VFB with a reference signal VREF. Logic circuit 103 comprises a first input coupled to the output of comparison circuit 102, a second input coupled to ON-time control circuit 101 to receive ON-time period control signal COT and an output coupled to a control terminal of the switch to provide a control signal CTRL.
When an equivalent series resistance (ESR) of an output capacitor is small, a sub-harmonic oscillation may occur at output signal OUT, and constant ON-time converter 100 may be unstable. A slope compensation circuit 105 may be employed to avoid the sub-harmonic oscillation. Slope compensation circuit 105 comprises an output configured to provide a slope compensation signal VSLOPE. Comparison circuit 102 is configured to provide the comparing signal via comparing a sum of slope compensation signal VSLOPE and feedback signal VFB with reference signal VREF.
But feedback signal VFB and output signal OUT may be influenced by slope compensation circuit 105 per the output of slope compensation circuit 105 is coupled to the output of feedback circuit 106 directly. As a result, output accuracy of constant ON-time converter 100 needs to be improved.